The present invention relates to the field of two-way wireless communications systems and more specifically to methods and apparatus using collector arrays in cellular sysems.
Cellular Systems
Present day cellular mobile telephone systems developed due to a large demand for mobile services that could not be satisfied by earlier systems. Cellular systems "reuse" frequency and other radio frequency (RF) resources within a group of cells to provide wireless two-way communication to large numbers of users. Each cell covers a small geographic area and collectively a group of adjacent cells covers a larger geographic region. Each cell has a fraction of the total amount of the RF spectrum or other resource available to support cellular users. Cells are of different sizes (for example, macro-cell or micro-cell) and are generally fixed in capacity. The actual shapes and sizes of cells are complex functions of the terrain, the man-made environment, the quality of communication and the user capacity required. Cells are connected to each other via land lines or microwave links and to the public-switched telephone network (PSTN) through telephone switches that are adapted for mobile communication. The switches provide for the hand-off of users from cell to cell as mobile users move between cells.
In conventional cellular systems, each cell has a base station with RF transmitters and RF receivers co-sited for transmitting and receiving communications to and from cellular users in the cell. The base station transmits forward channel communications to users and receives reverse channel communications from users in the cell.
The forward and reverse channel communications use separate channel resources, such as frequency bands or spreading codes, so that simultaneous transmissions in both directions are possible. With separate frequency bands, this operation is referred to as frequency division duplex (FDD) signaling. In time division duplex (TDD) signaling, the forward and reverse channels take turns using the same frequency band. In code division duplex (CDD), the signaling is spread across a wide spectrum of frequencies and the signals are distinguished by different codes.
The base station in addition to providing RF connectivity to users also provides connectivity to a Mobile Telephone Switching Office (MTSO) or Mobile Switching Center (MSC). In a typical cellular system, one or more MTSO's (MSC's) will be used over the covered region. Each MTSO (MSC) can service a number of base stations (which are also known as Base Transceiver Stations (BTS)) and associated cells in the cellular system and supports switching operations for routing calls between other systems (such as the PSTN) and the cellular system or for routing calls within the cellular system.
Base stations are typically controlled from the MTSO by means of a Base Station Controller (BSC). The BSC assigns RF carriers or other resources to support calls, coordinates the handoff of mobile users between base stations, and monitors and reports on the status of base stations. The number of base stations controlled by a single MTSO depends upon the traffic at each base station, the cost of interconnection between the MTSO and the base stations, the topology of the service area and other similar factors.
A handoff is a communication transfer for a particular user from one base station in one cell to another base station in another cell. A handoff between base stations occurs, for example, when a mobile user travels from a first cell to an adjacent second cell. Handoffs also occur to relieve the load on a base station that has exhausted its traffic-carrying capacity or where poor quality communication is occurring. During the handoff in conventional cellular systems, there may be a transfer period of time during which the forward and reverse communications to the mobile user are severed with the base station for the first cell and are not yet established with the second cell.
Cellular Architectures
In wireless systems, both physical channels and logical channels exist where logical channels carry signaling data or user data that is mapped onto physical channels. In cellular systems, traffic channels are logical channels for user data and are distinguished from control channels that are logical channels for network management messages, maintenance, operational tasks and other control information used to move traffic data reliably and efficiently in the system. In general, the term channels refers to logical channels unless the context indicates otherwise and those logical channels are understood to be mapped to physical channels. The control channels process the access requests of mobile users.
Conventional cellular implementations employ one of several techniques to allocate RF resources from cell to cell over the cellular domain. Since the power at a receiver of a radio signal fades as the distance between transmitter and receiver increases, power fading is relied upon to enable RF resource reuse in cellular systems. In a cellular system, potentially interfering transmitters that are far enough away from a particular receiver, and which transmit with acceptable transmission parameters, do not unacceptably interfere with reception at the particular receiver.
In a frequency division multiple access (FDMA) system, a communications channel consists of an assigned frequency and bandwidth (carrier). If a carrier is in use in a given cell, it can only be reused in other cells sufficiently separated from the given cell so that the other cell signals do not significantly interfere with the carrier in the given cell. The determination of how far away reuse cells must be and of what constitutes significant interference are implementation-specific details.
In a time division multiple access (TDMA) system, time is divided into time slots of a specified duration. Time slots are grouped into frames, and the homologous time slots in each frame are assigned to the same channel. It is common practice to refer to the set of homologous time slots over all frames as a time slot. Typically, each logical channel is assigned a time slot or slots on a common carrier band. The radio transmissions carrying the communications over each logical channel are thus discontinuous in time. The radio transmitter is on during the time slots allocated to it and is off during the time slots not allocated to it. Each separate radio transmission which occupies a single time slot is called a burst. Each TDMA implementation defines one or more burst structures. Typically, there are at least two burst structures, namely, a first one for the user access request to the system, and a second one for routine communications once a user has been registered. Strict timing must be maintained in TDMA systems to prevent the bursts comprising one logical channel from interfering with the bursts comprising other logical channels in adjacent time slots.
One example of a TDMA system is a GSM system. In GSM systems, in addition to traffic channels, there are four different classes of control channels, namely, broadcast channels, common control channels, dedicated control channels, and associated control channels that are used in connection with access processing and user registration.
In a code division multiple access (CDMA) system, the RF transmissions are forward channel communications and reverse channel communications that are spread over a wide spectrum (spread spectrum) with unique spreading codes. The RF receptions in such a system distinguish the emissions of a particular transmitter from those of many others in the same spectrum by processing the whole occupied spectrum in careful time coincidence. The desired signal in an emission is recovered by de-spreading the signal with a copy of the spreading code in the receiving correlator while all other signals remain fully spread and are not subject to demodulation.
The CDMA forward physical channel transmitted from a base station in a cell site is a forward waveform that includes individual logical channels that are distinguished from each other by their spreading codes (and are not separated in frequency or time as is the case with GSM). The forward waveform includes a pilot channel, a synchronization channel and traffic channels. Timing is critical for proper de-spreading and demodulation of CDMA signals and the mobile users employ the pilot channel to synchronize with the base station so the users can recognize any of the other channels. The synchronization channel contains information needed by mobile users in a CDMA system including the system identification number (SID), access procedures and precise time-of-day information.
Spread spectrum communication protocols include but are not limited to CDMA as well as Frequency Hopping and Time Hopping techniques. Frequency Hopping involves the partitioning of the frequency bandwidth into smaller frequency components, which a channel then uses by hopping from one frequency component to another in an essentially random manner. Interchannel distortion acts essentially as Gaussian white noise across time for each channel. Time Hopping involves a time division scheme wherein each channel starts and stops at differing time slots in an essentially random fashion. Again, interchannel distortion acts essentially as Gaston white noise across time for each channel.
Applicable communications protocols of this patent include but are not limited to FDMA, TDMA, and spread spectrum techniques, as well as protocols employing techniques of more than one of FDMA, TDMA and spread spectrum techniques.
Many cellular systems are inherently space division multiple access (SDMA) systems in which each cell occupies and operates in a zone within a larger region. Also, cell sectoring, microcells and narrow beam antennas all employ spacial divisions that are useful in optimizing the reuse of RF resources.
Space Diversity
The combining of signals from a single source that are received at multiple spaced-apart antennas is called space diversity. Micro-diversity is one form of space diversity that exists when two or more receiving antennas are located in close proximity to each other (within a distance of several meters for example) and where each antenna receives the signals from the single source. In micro-diversity systems, the received signals from the common source are processed and combined to form an improved quality resultant signal for that single source. Micro-diversity is effective against Rayleigh or Rician fading or similar disturbances. The terminology micro-diverse locations means, therefore, the locations of antennas that are close together and that are only separated enough to be effective against Rayleigh or Rician fading or similar disturbances. The signal processing for micro-diverse locations can occur at a single physical location and hence micro-diversity processing need not adversely impact reverse channel bandwidth requirements.
Macro-diversity is another form of space diversity that exists when two or more receiving antennas are located far apart from each other (at a distance much greater than several meters, for example, several kilometers) and where each antenna receives the signals from the single source. In macro-diversity systems, the received signals from the single source are processed and combined to form an improved quality resultant signal for that single source. The terminology macro-diversity means that the antennas are far enough apart to have de-correlation between the mean signal levels for signals from the single source. The terminology macro-diverse locations means, therefore, the locations of antennas that are far enough apart to achieve that de-correlation. Macro-diversity processing involves forwarding of signals to a common processing location and hence consumes communication bandwidth.
The mean signal levels in macro-diversity systems are de-correlated because each separate signal path has unique propagation properties that diminish the signal strength. The propagation properties in each path are different from those in each other signal path. These unique propagation properties vary with distances above Rayleigh or Rician fading distances and are due to terrain effects, signal blocking by structures or vegetation and other similar environmental factors. Fading due to such factors is referred to as shadow fading. De-correlation distances for shadow fading may be just above Rayleigh fading distances and may be as large as several kilometers.
User Location In Cellular Systems
In cellular systems, equipment and functions are distributed over zones, cells, and other coverage areas. In order to control and operate cellular systems efficiently, information about the location of active users in the system is increasingly important.
In conventional cellular systems, the user location information that has been used has included the cell, or sector of a cell, in which a user is located. The location of a user in a cellular system is important because of the fading of signals as a function of the distance of a receiver from a transmitter. Although increases in broadcast power can be used at greater distances between broadcasters and receivers, such increases tend to cause reception interference by other receivers and hence tend to reduce the user capacity of the system. Accordingly, cellular systems balance RF resources in order to optimize parameters that efficiently establish good system performance.
The problem associated with increases in capacity of cellular systems have created a need for improved methods and apparatus for use in wireless mobile communication systems.